In recent years, attention has paid to CSP (chip on size/scale package) techniques in LSI-mounting technology. Of these techniques, a package wherein lead terminals are taken in a package, a typical example of which is a QFN (quad flat non-leaded package), is one package form which is particularly notable from the viewpoint that the package can be made small-sized and the integration degree of semiconductor elements therein can be made high. Of processes for producing such QFNs, especial attention has been paid in recent years to a process of arranging a plurality of chips for QFNs in good order on die pads in package pattern areas of a lead frame, sealing them with a sealing resin, in a lump, inside a cavity made in a mold, and then cutting the resultant to be divided into individual QFN structures, thereby improving the productivity of the semiconductor elements per the lead frame area drastically.
According to such a process for producing QFNs, wherein plural semiconductor chips are sealed in a lump, the area which is clamped with a molding die when the chips are sealed with a resin is only the outside of a resin-sealing area which spreads to the outside of a package pattern area. Accordingly, in the package pattern area, in particular, at the center thereof, the outer lead face thereof cannot be pressed against the molding die with a sufficient pressure. Thus, it is very difficult to suppress the phenomenon that the sealing resin leaks out to the outer lead side. As a result, a problem that terminals of the QFNs are covered with the resin is easily caused.
Against a QFN-producing process as described above, therefore, a production process is known wherein a pressure-sensitive adhesive tape is adhered onto the outer lead side of a lead frame and at the time of sealing chips with a resin the leakage of the resin to the outer lead side is prevented on the basis of sealing-effect using the self-adhesive force of this pressure-sensitive adhesive tape (i.e., masking with the tape) (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2002-110884).
In this production process or the like, it is desired that the heat-resistant pressure-sensitive adhesive tape is adhered onto the outer pad face of the lead frame at the first stage and the adhesion is continued from the first stage, via the step of mounting semiconductor chips and the step of wire bonding, to the step of sealing the chips using the sealing resin. It is therefore necessary for the heat-resistant pressure-sensitive adhesive tape to have not only a characteristic that the adhesive tape prevents the leakage-out of the sealing resin but also characteristics satisfying all of the above-mentioned steps, for example, characteristics that the tape has a high heat resistance for resisting the step of mounting the semiconductor chips; the tape does not hinder delicate operation in the wire bonding step; and the tape can be satisfactorily peeled without leaving any paste after the end of the sealing step.
In recent years, disclosed was a process for producing the so-called leadless semiconductor device, wherein a copper foil is adhered onto a pressure-sensitive adhesive tape and then the foil is etched to form an electric conductor in order to make the electric conductor thinner (see, for example, JP-A No. 9-252014). According to this process, the electric conductor can be made thin since the conductor is formed on the tape. In the case of dividing the semiconductor device molded with a sealing resin into individual pieces, it is unnecessary to cut any lead frame. Therefore, at the time of dicing the semiconductor device with a blade, the blade is less worn away.
It is necessary for the heat-resistant pressure-sensitive adhesive tape used for such a purpose to have the same characteristics as in the case of using a lead frame, that is, to have not only a characteristic that the adhesive tape prevents the leakage-out of the sealing resin but also characteristics satisfying all of the above-mentioned steps, for example, characteristics that the tape has a high heat resistance for resisting the step of mounting semiconductor chips; the tape does not hinder delicate operation in the wire bonding step; and the tape can be satisfactorily peeled without leaving any paste after the end of the sealing step.
As a heat-resistant pressure-sensitive adhesive tape having characteristics as described above, a silicone based pressure-sensitive adhesive agent is generally used since the agent has excellent heat resistance and further has appropriate elastic modulus and adhesiveness.
However, it has been proved that when a silicone base pressure-sensitive adhesive agent is used as the heat-resistant tape, the following problems 1) and 2) are caused: 1) The silicone based pressure-sensitive adhesive agent of the heat-resistant pressure-sensitive adhesive tape is shifted to the outer pad portion and pollutes the surface thereof when the tape is peeled after the above-mentioned successive steps. As a result, when the semiconductor device is soldered onto the mounting substrate, the wettability thereof gets bad so that the mounting yield is lowered. 2) In order to obtain a good metal joint between wires and the lead pads in the wire bonding step, they may be heated to about 200° C. In the case of using the silicone based adhesive tape, siloxane gas is generated to pollute the surfaces of the lead pads. Additionally, a decline of the elastic modulus of the pads causes the wire bondability to be lowered.
In the case that a silicone pressure-sensitive adhesive agent is used in a leadless production process as disclosed in the above-mentioned JP-A No. 9-252014, the silicone pressure-sensitive adhesive agent is exposed to chemical agent solution in a wet process in etching and plating steps. Thus, eluted silicone components pollute the surface so as to cause a problem that the wire bondability of the surface is lowered.